William J. Wade

Diagenetic origin of carbon and oxygen isotope compositions of Permian-Triassic boundary strata

Bulk carbonate δ^(13)C and δ^(18)O compositions of profiles across Permian–Triassic (P–T) boundary sections in China, Italy, Austria, and Iran show wide varieties of trends. The δ^(13)C depletions occur in all sections and range from 2 to 8‰ PDB in magnitude. These excursions take place over intervals ranging from less than 0.1 to more than 40 m. The δ^(18)O values may increase or decrease toward the P–T boundary, but decrease sharply by 2–9‰ PDB at or above the boundary. Cross-plots of δ^(13)C and δ^(18)O values from all sections show positive covariance. Wide differences in magnitudes, trends, and position of the excursions relative to the boundary, as well as the covariance patterns suggest that P–T boundary δ^(18)O and δ^(13)C values are partially or entirely diagenetic in origin, formed in association with exposure surfaces. This interpretation implies that P–T boundary sections studied till date were subaerially exposed before, during, and after the mass extinction, resulting in the removal of strata containing key information about the extinction mechanism. This inference is consistent with the paleontological studies that have shown the presence of gaps at the boundary, and further supported by the sharp lithologic changes observed at virtually all P–T boundary sections. Subaerial exposures are documented by detailed sedimentologic and isotopic studies from central Tethyan sections in Abadeh and Shah Reza in Iran. Proposed P–T boundary extinction models are based on isotopic values that are diagenetic in origin and stratigraphic sections that are incomplete, leading to extinction mechanisms with little physical supporting evidence.

Massive Recrystallization of Low-Mg Calcite at High Temperatures in Hydrocarbon Source Rocks: Implications for Organic Acids as Factors in Diagenesis

We document massive recrystallization of low-Mg calcite lime mudstone source rocks at moderately high temperatures in the Smackover Formation of Mississippi. This process was driven by organic acids produced during kerogen maturation and holds the implications for studies of primary migration of hydrocarbons, secondary porosity generation in reservoirs, and global environmental change. This interpretation is based on contrasting the following petrographic and bulk-rock geochemical characteristics of organic-poor vs. organic-rich lime mudstones:

Meteoric Diagenetic Environment

Abstract Shallow-marine carbonate sequences commonly undergo exposure to meteoric waters. These waters are chemically aggressive toward sedimentary carbonate minerals, capable of rapidly dissolving grains and generating secondary porosity. The carbonate derived from dissolution can precipitate as cement, either nearby or hydrologically downstream, decreasing porosity. Thus the potential for restructuring of original depositional porosity is very high in the meteoric diagenetic environment. Chemical signatures of meteoric pore waters and meteoric carbonate cements are distinct and reflect kinetics of the CaCO 3 –H 2 O–CO 2 system, climatic effects, and hydrologic setting. The meteoric diagenetic environment is subdivided into vadose and phreatic diagenetic zones. Caliches/calcretes are distinctive diagenetic profiles of uppermost vadose zones in semi-arid climates. Porosity development in vadose diagenetic zones is to a large degree a function of relative sea level, which controls the occurrence of localized floating freshwater lenses (during highstands) versus regional meteoric water systems (during lowstands). Detailed examples presented include Quintana Roo (Mexico) strandplains and Oaks Field (North Louisian Jurassic), both highstand prograding shoreline systems, and Great Bahama Bank and Barbados (lowstand platform-wide aquifer systems). Geochemical trends in calcite cements and porosity development patterns characteristic of regional meteoric aquifer systems are illustrated from Mississippian Lake Valley Formation grainstones (southwest New Mexico). Karst processes and porosity styles are described in order that paleokarst features in reservoirs can be recognized and/or predicted. Detailed evaluations of paleokarsted reservoirs include Yates and Ellenburger fields (Permian and Ordovician of West Texas, respectively) and Rospo Mare Field (Cretaceous), Adriatic offshore, Italy. Lastly, the validity and significance of dolomitization associated with meteoric and especially mixed meteoric–marine waters (Dorag model) is evaluated and found to be lacking.

Illite polytype distribution as an inorganic indicator of thermal maturity in the Smackover Formation of the Manila Embayment, southwest Alabama

Abstract Illite polytype distribution is compared to pyrolysis Tmax values in the Smackover Formation in the Manila Embayment, southwest Alabama to determine its potential as an indicator of thermal maturity. The percent 2M illite polytype (2M/1M + 2M) increases from 20 to 80% over the formation temperature interval 100–165°C and as Tmax values increase from 432 to 461°C. Illite polytype distribution is considered a good qualitative indicator of thermal maturity in the Smackover Formation of the study area. The correlation between the percentage of 2M illite and temperature and thermal maturity in the Smackover Formation is believed to be primarily the result of the diagenetic transition from the 1M to the 2M polytype. Depositional effects are manifested in four samples as anomalously high 2M polytype percentages. This variation is interpreted to be the result of locally high concentrations of detrital 2M muscovite. Because original sediment composition and illite chemistry may vary significantly both locally and regionally, illite polytype distribution is a useful thermal maturity indicator only within a particular basin, and when calibrated with other thermal maturity indicators.

Natural Fracturing in Carbonate Reservoirs

Abstract Carbonate reservoirs are prone to natural fracturing. Fractures can act as enhanced permeability pathways, which may increase, decrease, or complicate reservoir production and development; healed fractures contribute to reservoir compartmentalization. A primary focus is placed upon the predictability of fracture set patterns and orientations, which vary according to carbonate lithofacies and the stress field(s) under which different types of fractures form. Extension fractures can form at the surface or at reservoir depths. Certain types of extension fracture sets (e.g., syndepositional, regional, and—to a lesser extent—karst-related fracture sets) exhibit predictable patterns and orientations with respect to the stress field under which they originated. Surface outcrops commonly exhibit multiple fracture sets; these are most frequently related to relaxation of compaction and/or thermal cooling. Such fracture sets are considered unlikely to resemble fracture sets in nearby reservoirs at depth; therefore, the use of surface fracture patterns as analogs for same-formation reservoirs, without comparative analysis of burial stress histories, is risky. Fault-related fractures have very high permeability potentials when newly formed, but their resulting role as fluid conduits typically leads to rapid healing, and therefore a higher likelihood of causing reservoir compartmentalization. These fractures typically cut across multiple beds. Fold-related fracture patterns are complex, typically consisting of both extension and conjugate shear-pair fractures, and show variable orientations in space and/or over time. However, they tend to follow the geometries of individual beds and are often confined to single beds, rather than aligning according to overall structural axes. Ekofisk Field, a naturally fractured North Sea chalk reservoir, is presented as an illustrative case of fold-related fracture abundance and effectiveness in enhancing fieldwide permeability parameters, without the drawback of creating major production problems during waterflooding.

Chapter 5 - Carbonate Diagenesis: Introduction and Tools✩

The three diagenetic realms in which porosity modifications (e.g., dissolution, cementation, compaction) take place are the marine, meteoric, and subsurface environments. The meteoric environment—with its dilute waters, easy access to CO2, and wide range of saturation states with respect to carbonate phases—has high potential for porosity modification, including destruction by cementation and generation of secondary porosity by dissolution. Modern shallow-marine environments are particularly susceptible to porosity destruction by cementation due to high levels of supersaturation of marine waters relative to metastable carbonate minerals. Decreasing saturation with depth can lead to development of secondary porosity by dissolution of aragonite. In the geologic past, shallow-marine waters were often undersaturated with respect to aragonite. The subsurface environment is marked by loss of porosity through compaction and related cementation. Thermal maturation and degradation of hydrocarbons and the slow flux of basinal fluids during progressive burial drive later porosity modification by cementation and modest local dissolution. Recognition and differentiation of the porosity modification history of carbonate rocks is aided by a number of analytical tools. Petrography enables us to reconstruct the sequence of relatively timed diagenetic events responsible for porosity modifications. Trace element and stable isotope analyses of cements and dolomites provide insight into the types of waters involved in these events. Two-phase fluid inclusions are used to estimate temperatures of cement or dolomite formation and the composition of precipitating or dolomitizing fluids. The definitiveness of trace element analysis is often limited by uncertainties in distribution coefficients, temperature fractionation effects, or low concentration values. Two-phase fluid inclusion studies also pose significant problems (e.g., stretching of inclusions during burial, recognition of primary inclusions, and accuracy of pressure corrections). Therefore, these tools should be used to provide constraints on assessing environments of diagenetic events, within an appropriate petrographic/geologic framework. The continuing development of new instruments and techniques (e.g., the ion probe, clumped isotope analysis) holds great promise for the future of geochemical analyses in diagenetic studies.

Chapter 9 - Summary of Early Diagenesis and Porosity Modification of Carbonate Reservoirs in a Sequence Stratigraphic and Climatic Framework✩

A comprehensive series of carbonate diagenesis/porosity models summarize the concepts developed in previous chapters, emphasizing the predictable loci of major porosity modification and enhancement. Each model refers to a specific combination of (1) setting (carbonate ramp, land-tied shelf, or isolated platform), (2) climate regime (humid or arid), and (3) sea-level cycle phase (TST, HST, or LST). Diagenetic processes at the parasequence scale reflect third-order sea-level cycles. During the TST and early HST, parasequences tend to be thick, with marine diagenesis dominating. Parasequences progressively thin during the HST, with exposure at cycle tops and meteoric influence becoming more important. During the late HST and the LST, subaerial diagenesis dominates. Third-order sedimentary sequences exhibit stacking geometries that reflect background second-order sea-level trends. Retrogradational sequence sets develop during second-order sea-level rise (e.g., in rift or foreland basins). Such sequence sets show relative domination by marine diagenesis. Aggradational sequence sets develop during second-order sea-level stillstand to moderate rise (e.g., early post-rift phase in extensional basins). Moderate meteoric water diagenesis and porosity modification occur at sequence boundaries, followed by burial diagenesis. Progradational sequence sets develop on passive margins during second-order sea-level stillstand to fall. This setting supports deep, amalgamated karstification, extensive phreatic meteoric diagenesis, and—under arid conditions—reflux dolomitization. First-order Icehouse conditions are characterized by high-frequency, high-amplitude sea-level cycles that favor development of rimmed carbonate shelves. The mainly aragonitic sediments deposited on these aggraded shelves experience high degrees of meteoric diagenesis and porosity modification. Greenhouse conditions are characterized by lower-frequency, low-amplitude sea-level cycles that favor development of carbonate ramps. The calcite sediments deposited here result in relatively muted meteoric diagenesis and porosity modifications. Two case histories illustrate the basic concepts of early diagenetic porosity evolution: (1) the Southwest Andrews Area, an Icehouse Permian–Pennsylvanian rimmed shelf margin reservoir (Permian, West Texas), and (2) ramp sequences of the Kwanza and Lower Congo basins, Greenhouse Albian Pinda Group (Cretaceous, offshore Angola).

Chapter 12 - Case Histories✩

Three economically important case histories serve as illustrations of the integration of analyses of depositional environments, sequence stratigraphic architecture, and porosity evolution during diagenesis, as a means of maximizing effectiveness of reservoir production and/or modelling: (1) the Paleozoic Madison Formation of central Wyoming, (2) the Upper Jurassic Smackover Formation of the central Gulf of Mexico, and (3) the Tertiary Malampaya buildup, offshore Philippines. The three embody a broad range of geologic contexts (e.g., icehouse versus greenhouse during deposition) and different approaches for optimizing development programs (e.g., use of surface analogs, 3D seismically based reservoir modelling). High drilling costs during development of the deep (23,000 ft.) Madden Field in the Wyoming Madison Formation (due to high temperature, pressure, and H2S content of the gas) mandated high efficiency during development. Meticulous evaluation of a surface outcrop analog and maximized collection of analog data were the primary means of assuring optimal reservoir development. The Upper Jurassic Smackover trend in the central Gulf of Mexico illustrates revitalization of a mature petroleum fairway through application of sequence stratigraphic interpretation. Previously overlooked lowstand siliciclastic slope fans become geographically and stratigraphically predictable reservoir targets when understood in their proper sequence stratigraphic framework. The 3D seismic grid over the drowned isolated Oligocene–Miocene Malampaya platform, offshore Philippines, is integrated with geologic and petrophysical data from sparse well control and field-wide depositional and diagenetic models in order to develop a reservoir simulation model of the reservoir.

Chapter 10 – Burial Diagenetic Environment ✩

The overarching diagenetic drive during progressive burial of carbonate rocks is toward the loss of porosity through mechanical and chemical compaction (the latter consisting of pressure solution plus related cementation). The passive margin diagenetic regime is marked by relatively rapid burial with steadily rising temperatures and pressures. Once a mechanically stable grain framework is achieved, the effective stress from sediment loading can eventually suffice to cause chemical compaction. Early cementation, the presence of organic frameworks, overpressuring, dolomitization, and especially the filling of reservoir pores with oil all act to retard the onset and efficiency of chemical compaction. Aggressive pore fluids, the presence of metastable mineral phases, and admixtures of siliciclastics or other insolubles tend to accelerate the process. Catagenesis of organic matter in source rocks yields aggressive formation waters capable of calcite dissolution just prior to hydrocarbon maturation. But despite evidence of local late secondary porosity generation, theoretical considerations lead to the conclusion that such evidence represents limited and local porosity rearrangement. Hydrocarbon-filled deep carbonate reservoirs experience progressive loss of porosity with increasing depth, due to precipitation of pyrobitumen as circumgranular linings, and also—during very deep burial—by renewed precipitation of calcite cement (with carbon derived from destruction of methane) and resumption of mechanical and/or chemical compaction. The active margin diagenetic regime is characterized by rapid movement of large volumes of warm-to-hot basinal fluids, mobilized by tectonism, through complex subsurface conduits. Reactions between expelled fluids and conduit carbonates include recrystallization of earlier dolomites and calcites, replacement dolomitization, dissolution of calcite, dolomite and/or evaporites, and sulfide mineralization. Ascending hydrothermal fluids associated with wrench faults and rifting may result in fault-localized leaching of calcite, dolomitization, and dolomite cementation capable of creating or enhancing “hydrothermal dolomite” reservoirs. The most abundant diagenetic product is saddle dolomite. The postorogenic diagenetic regime is characterized by topographically driven meteoric recharge into deeply buried aquifers. There is minimal impact on carbonate porosity unless meteoric waters are dissolving anhydrite/gypsum. Where this occurs, a chemical drive can promote dissolution of dolomite and precipitation of calcite, accompanied by porosity enhancement. In the absence of evaporite dissolution, meteoric waters equilibrate with carbonate aquifers and minimal porosity modification occurs downstream. Crossplots of porosity versus thermal maturity appear to possess porosity predictive capability. Carbonate reservoirs are relatively prone to souring at depth; hydrocarbons in sour reservoirs can be partially or entirely consumed by destructive redox reactions with sulfur in the forms of H2S and S0. The initiation and extent of these reactions depend upon the availability of reactant sulfur (from evaporites and organosulfur compounds) and dissolved iron (derived from Fe-rich siliciclastics, if proximal).

Evaporative Marine Diagenetic Environment

Abstract Two marine evaporative settings are presented in detail: the sabkha and the evaporative lagoon/salina. In each, diagenetic pathways affect porosity evolution in associated marine carbonate sequences, with common dolomitization being a principal factor. Dolomitization is favored where hypersaline waters possess high Mg/Ca ratios (postprecipitation of Ca-bearing evaporites) and potential for hydrologic drive (high fluid densities). Surficial dolomites in modern environments are poorly ordered “protodolomites”. Modern marginal marine sabkha diagenetic environments are thin ( In contrast, reflux dolomitization adjacent to evaporative lagoons can impact large volumes of grainy carbonates, with superior petrophysical parameters. The following examples of ancient reflux dolomitization are presented: Guadalupian of the Permian Basin (west Texas) and Upper Jurassic Smackover Formation (east Texas). Additional examples of ancient evaporative lagoons and salinas linked to important reservoirs (dolomitized or not) are discussed, including the Elk Point Basin (western Canada), Michigan Basin, and Arab Formation giant reservoirs of the Middle East.